This work presents a comprehensive study of the fabrication process of the sol-gel derived zinc oxide (ZnO) buffer layer in solution processed inverted bulk heterojunction organic solar cells based on a thin film blend of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b: 3,4-b']dithiophene-2,6-diyl]] and [6,6]-phenyl C71 butyric acid methyl ester. The fabrication process parameters, such as the zinc acetate concentration ratio of the precursor solution, annealing temperature and annealing time of the ZnO film, were investigated in this work. The ZnO fabrication process influences both the quality of ZnO film and the contact between the ZnO buffer layer and the active layer. Optimization of the processing parameters caused the short circuit current density to increase from 4.7 to 11.2 mA/cm(2) and the fill factor increases from 28.8% to 42.5%. This led to an increase of the power conversion efficiency from 0.11% to 3.43%. The result indicates that the photovoltaic performance of inverted structure organic solar cells is strongly dependent on the optimized sol-gel derived ZnO film. (C) 2014 Elsevier B. V. All rights reserved.